What does the sphere of my prescription say about my visual acuity?

Near-, or farsighted: If the first number (the sphere) on your prescription has a + notation before it, you are hyperopic (or farsighted), which means you can see better far away than up close (relatively). If there is a - in front of that first number, then you are myopic (nearsighted) which means you see better up close than far away, relatively.

Overall prescription: The eye is essentially a camera with 2 lenses. Neither lens is perfectly round, although in most people they are close. Depending on the shape and size of your eye and lenses, you may be nearsighted (see better up close), farsighted (see poor up close and possibly far away also), and/or have astigmatism (blur at any distance). The sphere is the correction for your near- or farsightedness only.

Near or far sighted: Sphere is the degree of near (minus power) or far (plus power) sightedness. For example, -3.00 sphere is 3 diopters of nearsightedness. -4.00 would be even more nearsightedness. Put a + in front of those numbers instead, and that would be farsightedness. Sphere does not include any astigmatism, a.K.A. Cylinder.